Anti-waveguide routing structure

ABSTRACT

In a first state of an optical switch, a structure in the switch confines an optical mode to propagate along a first, unswitched path. The switch is switched into a second state by reducing the refractive index along the first path, or by increasing the refractive index of a region of the switch outside but adjacent to the first path, until the index within the first path is lower, preferably substantially lower, than that of the adjacent region. This creates an anti-waveguiding section in which light is forced to diverge from the unswitched path both by diffraction and refraction. The refractive index change is produced thermo-optically or electro-optically, for example. In a symmetric planar embodiment, upon actuation, light escapes from the confinement region into two beams deflected symmetrically in lateral directions while remaining vertically confined. In an asymmetric planar embodiment, upon actuation, light from the confinement region escapes in one direction away from the confinement region in the horizontal plane, while remaining confined vertically and in the opposite direction in the horizontal plane. A self-aligned method for fabricating optical switches is also described.

[0001] This is a Division of U.S. patent application Ser. No.09/630,511, filed Aug. 2, 2000, incorporated by reference herein.

[0002] This invention was made with Government support under ContractNo. DAS60-96-C-0149 awarded by the Department of Defense. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The invention relates to integrated optics, more particularly, toswitching and redirecting of light beams in optical waveguides.Manipulation of waveguide modes, which are light beams inside integratedoptical devices, is a powerful capability that has useful applicationsprimarily but not exclusively in displays and telecommunications, suchas distribution of light in flat panel displays, re-routing ofWavelength-Division Multiplexed communications channels, and otherintegrated optics switching tasks.

[0005] 2. References

[0006] The following documents are incorporated herein by reference:

[0007] 1) U.S. Pat. No. 5,544,268, August 1996, Bischel et al, “Displaypanel with electrically controlled waveguide-routing”

[0008] 2) U.S. Pat. No. 3,801,185, April 1974, Ramaswamy et al, “Switchfor thin-film optics”

[0009] 3) U.S. Pat. No. 5,009,483, April 1991, and U.S. Pat. No.5,106,181, April 1992, Rockwell III, “Optical waveguide display system”

[0010] 4) J. Viitanen and J. Lekkala, “Fiber optic liquid crystaldisplays”, SPIE Vol. 1976, High Definition Video, pg. 293-302 (1993)

[0011] 5) U.S. Pat. No. 5,045,847, September 1991, Tarui et al, “Flatdisplay panel”

[0012] 6) R. Akkari et al., “Thermo-optic mode extinction modulation inpolymeric waveguide structures”, Journ. Non-Cryst. Solids, 187, pg.494-497 (1995)

[0013] 7) U.S. Pat. No. 4,648,687, Yoshida.

[0014] 3. Description of Related Art

[0015] Whereas in conventional optics, light beams are switched andmanipulated by placing discrete optical elements such as mirrors,lenses, prisms, and etalons in their path, in integrated optics thesefunctions are performed much more compactly, inside solid materialstructures that contain both optical waveguides and the means formanipulating the light beams traveling therein. The term “light” as usedherein denotes “electromagnetic energy” and “optical energy” or “opticalpower” in general without restriction to visible wavelengths. It isknown in the art as a basic principle that refractive index differencesand gradients are key to guiding of light waves as well as to theirmanipulation, in solid materials. An optical waveguide, well known inthe art, is an elongated region of material with a higher index ofrefraction called a core, typically surrounded by material with a lowerindex of refraction called a cladding, both materials being opticallytransparent to a greater or lesser degree. The core is laterally narrowbut elongated along the desired path of light travel. Light travelingwithin such a structure becomes concentrated or confined to the higherindex region, in a spatial intensity distribution that is called anoptical waveguide mode. It is understood in the art that, as a guidedlight beam encounters different magnitudes of the refractive indexarranged in different geometric shapes in the material, its mode shapechanges in spatial extent, local intensity, and direction of travelaccordingly. In known devices the electro-optic effect with electricalactuation, and the thermo-optic effect with actuation by an electricalheater element or by heating due to optical absorption of light emittingdiode or diode laser light of suitable wavelength, have been employed toproduce refractive index changes in response to an applied controlsignal.

[0016] Several techniques are known and have been used in the prior artof integrated optics for switching and redirecting light beams inoptical waveguides.

[0017] Waveguide switches called TIR (Total Internal Reflection) or PIR(Partial Internal Reflection) switches, for routing of light beams toilluminate pixels in a display, are described in Bischel U.S. Pat. No.5,544,268. The switches are based on electro-optically,thermo-optically, acousto-optically or magneto-optically creating alower refractive index discontinuity situated at an angle to thewaveguide direction so as to cause total internal reflection or partialinternal reflection of the guided beam through a significant angle, uponapplication of a voltage or heat.

[0018] Waveguide switches variously called cut-off modulators,mode-extinction modulators, or cut-off switches, are described in otherreferences. These devices are based on decreasing the refractive indexdifference between core and cladding of a section of optical waveguidebelow a value required for local confinement and guiding of light, atthe wavelengths of interest in an application, by means of theelectro-optic, thermo-optic or acousto-optic effect. If the refractiveindex difference is decreased below this value, all wavelengths becomeunguided (guiding becomes “cut off”), as the material is thensubstantially uniform with no local confinement of light. In terms ofelectromagnetic field theory, guided wave propagation is cut off and thelight beam spreads out by diffraction, at a rate determined by theguided beam diameter entering the cut-off region, smaller modesgenerally diverging faster.

[0019] Most of the prior art switches and modulators suffer from severalshortcomings including high drive power requirement, large size on anintegrated optics chip, and inability to control the angle at whichlight is coupled out in the waveguide cut-off state. The presentinvention overcomes these problems by employing a different type ofrefractive index change, thereby providing a much smaller and moreefficiently actuated device with more control over the out-couplingangle.

SUMMARY OF THE INVENTION

[0020] The invention provides an anti-waveguide routing structure, alsocalled a waveguide switch or mode manipulator, that is a section of anotherwise permanent optical waveguide with a controllable refractiveindex change which exceeds that required to merely suppress waveguiding,thereby forcing a sharp redirection of the light beam from its originaldirection into selected transverse directions.

[0021] According to the invention, roughly described, an optical switchhas a first state and a second state. In the first state, a structure inthe switch confines one or more optical modes to propagate along afirst, unswitched path. The switch is switched into the second state byreducing the refractive index along the first path, or alternativelyincreasing the refractive index of a region of the switch outside butadjacent to the first path, until the index within the first path islower, preferably substantially lower, than that of the adjacent region.This creates an anti-waveguiding section in which light is forced todiverge from the unswitched path within a short distance, and into adesired transverse direction, by a combination of diffraction, owing toremoval of waveguiding confinement; and refraction, owing to bending ofthe wavefront toward a higher-index region, and in some embodimentsreflection from a high-low index boundary surface. The refractive indexchange is produced by known means such as the thermo-optic effect, theelectro-optic effect, or other suitable effect such as magneto-optic oracousto-optic, together with its associated actuation means known in theart such as a thin-film electrically powered element for heating, orheating provision by a light beam, or electrodes for applying anelectric field.

[0022] Particular selected transverse directions of weak confinementlead to planar, asymmetrical planar, and vertical versions of theanti-waveguide routing structure. The planar version has weakconfinement laterally in the horizontal plane, equally on both sides ofthe confinement region, and upon actuation, light is forced out of theconfinement region into two beams that are deflected symmetrically inlateral directions while remaining vertically confined. The asymmetricalplanar version may have weak confinement only on one lateral side of theconfinement region, and upon actuation, light from the confinementregion is forced preferentially into a second path in the horizontalplane, while remaining vertically confined. Asymmetrical planaroperation can also be realized in a structure with horizontallysymmetrical weak confinement but an asymmetrically positioned actuationmeans such that, upon actuation, an anti-waveguide region is createdprimarily to one side of the center line of the waveguide. A verticalembodiment has weak confinement at a lower cladding layer located on theopposite side of the core from the actuation means, but strongerconfinement in the other transverse directions, such that uponactuation, light from the confinement region is forced into a secondpath that diverges away from the actuation means, and generally in thedownward direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention will be better understood upon reference to thefollowing detailed description in connection with the accompanyingdrawings, in which:

[0024]FIG. 1 is a symbolic perspective view of an anti-waveguide routingstructure incorporating features of the invention.

[0025]FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1.

[0026]FIG. 3 is a cross-sectional view of a ridge waveguide embodimentof the invention.

[0027]FIG. 4 shows lateral light intensity profiles for differentdegrees of actuation, during operation.

[0028]FIG. 5 is a flow chart describing a fabrication procedure for aridge waveguide embodiment of a planar anti-waveguide routing structure.

[0029]FIG. 6 is a symbolic perspective view of a thermo-opticapplicator.

[0030]FIG. 7 is a chart showing variation of the deflection angle withstrength of anti-waveguiding.

[0031]FIG. 8 is a plan view of an asymmetrical embodiment of theinvention, showing an asymmetrically positioned applicator ofnon-uniform width.

[0032]FIG. 9 is a cross-sectional view of an asymmetrical planarembodiment of the invention, taken along line C-C in FIG. 8.

[0033]FIGS. 10 and 11 are cross-sectional views of electro-opticallyactuated embodiments of the invention.

[0034]FIGS. 12 and 13 are flow charts describing self-alignedfabrication procedures incorporating features of the invention.

[0035]FIG. 14 is a plan view of an asymmetrical embodiment of theinvention, including a target structure.

[0036]FIG. 15 is a chart showing variation of the temperature and therefractive index in the vertical direction through a vertical embodimentof the invention.

DETAILED DESCRIPTION

[0037] Referring to FIG. 1 and to a cross-sectional view FIG. 2 takenalong the line A-A, there is shown an anti-waveguide routing structure100 comprising a core 110, an upper cladding layer 120, a lower claddinglayer 130, side cladding layers 140 and 150, an applicator 160 that issituated on the upper cladding layer 120 directly above a section of thecore 110, and electrical connections 170 to a control unit 190. Controlunit 190 is only symbolic, and could in various embodiments be made upof one or more sub-units spread over a small or large area, and can beimplemented in software, hardware, or a combination of both. The controlunit 190 is intended to include both the drive electronics and anycontrol intelligence that might be required for a particularapplication. The materials of construction of the structure 100 are suchthat in the normal condition, that is, without activation of theapplicator, the refractive index of the core is larger than that of thecladding layers, and the parts 110, 120, 130, 140, and 150 comprise anoptical channel waveguide according to known art, where propagation oflight is substantially confined to the core, in a normal waveguide mode,as indicated generally by 280. The materials have the further propertiesthat, in the normal condition, the refractive indices of the upper andlower cladding layers 120 and 130 are significantly smaller than therefractive index of the core 110, thereby providing strong confinementvertically according to known art, and the refractive indices of the twoside cladding layers 140 and 150 are equal to each other and only alittle smaller than the refractive index of the core, thereby providingweak lateral confinement of light in a normal waveguide mode. The effectof strong vertical and weak lateral confinement is to limit theredirection of light upon actuation to the horizontal plane whilekeeping light confined in the vertical direction, and therefore a devicewith such further properties is herein called a planar anti-waveguiderouting structure.

[0038] The applicator 160 is shown to be a thin-film heater that is aresistive element activated by an electric current connected throughwires 170 from control unit 190. The material of the core 110 hasfurther thermo-optic properties known in the art whereby a change intemperature (due to the applied heat) produces a change of refractiveindex in the material depending on the local temperature. The amount ofchange is expressed by the thermo-optic coefficient herein termed dn/dT.The core material is chosen to have a large negative dn/dT, such asprovided by the polymers PMMA or polyurethane, so that ahigher-temperature portion of the core nearer to applicator 160 whenactivated will have a substantially smaller refractive index than alower-temperature part situated farther away from the applicator. Thematerial of the cladding regions on the other hand may be, but do nothave to be, chosen to be less affected by temperature or alternatively,affected in the opposite direction. However, the top cladding materialis chosen preferably to have a large refractive index difference withrespect to the core layer, to minimize the insertion loss from theapplicator. The lower cladding is less critical and is chosen to have arefractive index lower than the core layer. Examples of suitablematerials include glasses (for example Corning 1734), polyacrylates (forexample Gelest UMS-182), urethanes, and polyimids. Particular materialsare mentioned here for illustrative purposes and it should be apparentto those skilled in the art that other materials may also be foundsuitable for the functions described.

[0039] Activation of the applicator thus produces a spatially varyingchange of refractive index, superimposed on an otherwise permanentoptical waveguide. Upon activation, the refractive index of at least aportion of the core 110 becomes substantially reduced relative to therefractive index of a part of the side cladding regions 140 and 150adjacent to the core, but not relative to the upper and lower claddinglayers. The resulting index profile is spatially continuous, althoughnot necessarily uniform. As used herein, one region is considered“adjacent” to another region if it is within such a distance as toinfluence the propagation of the optical mode in an otherwise permanentwaveguide, which generally implies within a few evanescent decay lengthsof the mode. The effect of the decreased refractive index of at least aportion of the core, relative to the refractive index of the regionslaterally outside and adjacent to the core, is to force light out of theunswitched waveguide within a short distance, and into two laterallydiverging paths in the horizontal plane, by the sum of two well-knownprocesses of optics: diffraction and refraction. Light spreads bydiffraction, owing to removal of guided wave confinement laterally; anddeflects by refraction, owing to bending of the wavefront away from thesmaller refractive index region created in the core, and toward the nowrelatively larger-refractive index cladding laterally outside the coreregion. As used herein, an “anti-waveguide” is a structure in which theoptical index profile is such as to force optical energy to diverge froma normal propagation path both by diffraction and by refraction toward ahigher index adjacent cladding region.

[0040] The operation of an anti-waveguide routing structure is madeclearer with reference to FIG. 4, which depicts lateral light intensityprofiles of an optical mode resulting from a planar anti-waveguiderouting structure, under different degrees of actuation. The profilesschematically represent the variation of light intensity as a functionof distance from a vertical line of symmetry 290, measured in thelateral direction along a horizontal line X-X passing through the core(FIG. 2), in a cross-sectional plane cutting a line B-B identified inFIG. 1, which is located at some distance downstream of the applicatorin the unswitched waveguide propagation direction. The degree ofactuation affects the magnitude of the lateral refractive indexdifference between the core and the side cladding adjacent to the core,which in turn affects the strength of waveguiding or anti-waveguiding oflight in the structure. Curve 410 is for the unswitched condition, thatis, without activation of the applicator, in which the refractive indexof the core is larger than that of the side cladding adjacent to thecore, resulting in an optical waveguide mode that is confined within thecore, with little light intensity outside the same. Curve 420 is for asmall degree of actuation, corresponding to a reduction of therefractive index of the core to be approximately equal to that of theside cladding adjacent to the core, and resulting in a wider mode withlight spreading out beyond the core. Here the waveguide is near nominaleffective cut-off, where the peak intensity of light is still highest inthe original direction of propagation, but the magnitude is decreasingwith longitudinal distance because of spreading by diffraction. Curve430 is for a medium degree of actuation, corresponding to a reduction ofthe refractive index of the core significantly below that of the sidecladding adjacent to the core, and resulting in a mode with twosymmetrical lobes or peaks 431, propagating at an angle to the originaldirection, with little light intensity remaining in the region of theprior waveguide mode. Curve 440 is for a large degree of actuation,corresponding to a further reduction of the refractive index of the corewell below that of the side cladding adjacent to the core, and resultingin a mode profile 440 with peaks 441 propagating at a still larger angleof deviation from the original direction, as indicated by the distances432 for medium actuation and 442 for strong actuation. With increasingdegree of actuation, the deviation angle increases, and the mode peaksmove farther out laterally, at a given longitudinal position along thewaveguide.

[0041] It is apparent from the foregoing description that, atcross-sections taken further downstream from the applicator, the twosymmetrical peaks of an optical mode corresponding to a given actuationstrength within the anti-waveguiding range will be situated at furtherdistances laterally from the line of symmetry, because of deflection atan angle. On the other hand, the single peak of an optical modecorresponding to weaker actuation, before and up to nominal effectivecut-off, will still appear centered on the line of symmetry but it willbe of smaller amplitude and laterally wider than the guided mode profilebecause of diffraction.

[0042] An alternate embodiment of a planar anti-waveguide routingstructure is shown in cross-sectional view in FIG. 3, that is a ridgewaveguide structure comprising an extensive slab core layer 340 with aridge 310 that is a thicker section of the same material, to providelateral confinement in place of the side cladding layers 140, 150, anddiscrete core region 110. It is known in the art that light is laterallyconfined to the thicker core region at the ridge in such a structure, asindicated by 380. Lateral confinement is understood in terms of an“effective” refractive index per vertical slice through the structure,as described for example in Marcuse, D., Theory of Dielectric OpticalWaveguides, Second Edition, Academic Press, Inc., San Diego, 1991,incorporated by reference herein. The region immediately laterallyoutside the ridge has a smaller effective refractive index than theridge, owing to more smaller-index cladding material 330 and lesslarger-index core 340 material, per vertical slice.

[0043] It is apparent to those skilled in the art that the descriptionof operation of an anti-waveguide routing structure with reference to aplanar embodiment shown in FIG. 2 will apply equally to a planar, ridgewaveguide embodiment shown in FIG. 3 with substitution of the terms“ridge region” for “core” and “region immediately laterally outside theridge” for “side cladding adjacent to the core”.

[0044] It should now be further apparent that the present inventionemploys a refractive index change that is different in magnitude andgeometry from that known in prior art, to enable a new type of lightdeflector or redirector using refraction in addition to diffraction,thereby providing a greatly improved functionality. Some of theadvantages of the invention over prior art are: higher efficiency interms of required activating power to achieve a given extinction ratioin the active or on-state; lower insertion loss in the off-state of awaveguide switch upon aging; and less sensitivity to misalignment infabrication.

[0045] The anti-waveguide routing structure of this invention mayrequire less electrical power to achieve a given efficiency ofredirection of light than a TIR switch, because a thermo-opticapplicator in the structure will occupy less area than in the switch.Both devices can use the thermo-optic effect to create a region of lowerrefractive index and can require approximately the same degree ofactuation in terms of the refractive index change and heating powerdensity, expressed in units such as Watts per square centimeter. TIRswitches in prior art typically operate with a thermo-optic applicatorthat is a heater placed at an angle of a few degrees with respect to thewaveguide, crossing over the waveguide and extending approximately 10-20μm in the perpendicular direction on either side of the waveguide axis,in order to ensure that the entire optical field is deflected duringswitch activation. Waveguides with larger mode widths may require theheater to extend even more than 20 μm on each side of the waveguide. Thealternative use of a shorter heater situated at a larger angle to thewaveguide would not significantly reduce the power requirement of a TIRswitch, because increasing the intersection angle between the applicatorand waveguide increases the refractive index change required to achievea given reflection efficiency and therefore once again increases thepower density required at the heater. The required power densityincrease offsets the heater area decrease owing to a shorter length,resulting in a higher product of power density and heater area. In theembodiments of the present invention described herein, on the otherhand, the applicator is significantly narrower, not extendingsubstantially beyond the core as indicated in FIG. 2, and thus has lessarea and consequently requires less power. The applicator here acts toreduce the refractive index in the core at the central part of theunswitched waveguide mode, and thereby creates anti-waveguiding withoutthe need to act across the entire mode profile including the evanescenttail regions.

[0046] An anti-waveguide routing structure may possess a betterextinction ratio compared to cut-off modulators. The extinction ratio isthe light intensity measured at the output of a routing structure ormodulator when actuated, divided by the light intensity measured at theoutput when it is in an off-state. A large extinction ratio is desirablefor a switch. The output of such a device may be situated at the outputfacet of an unswitched waveguide section downstream of an applicator. Asillustrated in FIG. 4, there is a significant qualitative differencebetween an anti-waveguiding optical mode shape, obtained withsufficiently large actuation, and that for mere nominal effectivecut-off obtained with a small degree of actuation as described in priorart. The deflected lobes of an anti-waveguide routing structure haveminimal intensity in the unswitched waveguide direction, while the modepeak of a cut-off modulator on the other hand is still centered in theunswitched waveguide direction and decreased in amplitude only byexpansion of a single mode by diffraction. Consequently in theanti-waveguiding structures described herein there is much less lightpropagating in the unswitched direction at the output facet whenactivated, than in a conventional cut-off modulator, yielding a largerextinction ratio than in conventional devices.

[0047] Polymer thermo-optic devices are sometimes subject to drift ofoptical properties, such as a permanent refractive index change, causedby repeated temperature cycling in normal operation. The magnitude ofsuch change will be greatest in close proximity to the heater. In a TIRswitch, any such change will occur along a shallow-angle TIR/PIRinterface and cause permanent light deflection from the waveguide,resulting in an insertion loss in the off-state. However, in ananti-waveguide routing structure the boundary of any such permanentrefractive index change can be made to be substantially perpendicular tothe waveguide direction and thus results in significantly less Fresnelreflection than the glancing angle of incidence in the TIR structure andconsequently smaller insertion loss. For example, for a permanentrefractive index change of 0.0005, a 1.5-degree TIR switch acquires a0.2 dB insertion loss, while an anti-waveguide routing structuredevelops an insertion loss of 0.02 dB, which is an order of magnitudesmaller. Low insertion loss is especially desirable in applicationswhere an incoming light beam must propagate through many waveguideswitches or routing structures.

[0048] For use in display applications the anti-waveguide routingstructure as described herein is preferable over a TIR switch, asalignment of the applicator is much less critical than in a TIR switch.In a TIR switch, a misaligned applicator can produce displacement of theeffective pixel location. In the devices described herein, on the otherhand, the principal effect of misalignment is only a reduced efficiencyin terms of the electrical activation power required, without change ofdeflection angle or pixel location.

[0049] A first planar embodiment of the invention is a thermo-opticallyactuated ridge waveguide structure, described with reference to FIG. 3,and further described here in greater detail. The fabrication of theembodiment is best explained also with reference to the flow diagram ofFIG. 5, which shows an example of one of many fabrication methods thatmay be employed.

[0050] The structure may be fabricated on top of a substrate, not shown,which is preferably a glass wafer but may be any material of sufficientthickness to provide mechanical support for subsequently depositedlayers, for example a metal, polymer, semiconductor, or glass such asCorning 1734. For applications requiring flexibility, a polymersubstrate is preferred over rigid substrates. If no substrate isprovided, a lower cladding layer 330 of appropriate thickness may act asthe substrate.

[0051] In the first step 510 of the fabrication procedure shown in FIG.5, a layer of material, is deposited on the substrate surface for thepurpose of increasing adhesion of the cladding layer, which ispreferably a polymer. The desired characteristics of such an adhesionlayer are good adhesion and chemical compatibility with the materials oneither side of it, without chemical reaction and change in propertiesover time. The adhesion layer is preferably Norland Optical Adhesive 68,deposited preferably by spinning which is well known in the art.

[0052] In the second step 520 of the fabrication procedure, an opticallytransparent lower cladding layer 330 is deposited on the layer ofmaterial (the adhesion layer), to a preferred thickness of 5.0 μm,preferably by spinning. The lower cladding layer 330 is preferably apolyacrylate copolymer (for example Gelest UMS-992), with refractiveindex of 1.49 and a small dn/dT of approximately −1.8×10⁻⁴/° C. Astandard polymer layer deposition procedure is employed, comprisingviscosity adjustment prior to deposition, spinning, and curing withultraviolet light. In the polymer procedure, prior to deposition,approximately 0.5% of a photo-initiator (for example Daracor 1178) isadded to the cladding material, along with a suitable quantity ofsolvent to adjust viscosity, to enable the layer to be deposited at areasonable spin speed. Different viscosity adjusting solvents areappropriate for different polymers, and for the lower cladding layer itis preferably PGMEA. After spinning, the layer is cured under nitrogenusing ultraviolet light in the wavelength range of 250 nm to 450 nm, topromote crosslinking of the polymer chains and create a chemically andmechanically robust layer. Suitable cure times are approximately 1minute with 100 W lamp power at 250 nm wavelength, or 5 minutes with 100W at 400 nm. Alternatively another optical material may be used for thelower cladding layer, such as another type of polymer, sol-gel,aero-gel, liquid crystal, semiconductor, silica glass, or ceramic; andalternatively deposited by another method known in the art, such asmeniscus, extrusion, spray, dip, evaporation coating, or sputtering.

[0053] In the third step 530, inverted ridges 310 are etched into thelower cladding layer 330, to a preferred depth of 0.0650 μm. The ridgesare defined using standard photo-lithographic techniques, by depositinga positive photoresist layer on the lower cladding layer, exposing onlythe ridges through a mask, developing and removing the exposedphotoresist in the ridge areas, and etching to create the ridgespreferably by means of reactive ion etching (RIE), known in the art,while the rest of the lower cladding layer surface remains protected byphotoresist. The mask defining the waveguide may be aligned with respectto marks lithographically patterned on the substrate, to provideregistration between the waveguide and subsequent patterned features.For etching the preferred lower cladding material, a polyacrylatecopolymer, RIE in a trifluoromethane (CHF₃) gas plasma is suitable, withan etch time of a few minutes at a power of approximately 25-50 W.Alternatively a chemical etching process known in the art may beemployed in place of RIE. The remaining photoresist is then exposed andremoved. Design calculations to determine the preferred depth and widthof the ridges are known in the art and described, for example, inNishihara et al., Optical Integrated Circuits, McGraw Hill, 1989,incorporated by reference herein.

[0054] In the fourth step 540, a core layer 340 having a refractiveindex greater than both cladding layers 320 and 330, and having a largenegative dn/dT, is deposited on top of the now patterned lower claddinglayer 330, to a preferred thickness of 1.2 μm. The core layer ispreferably Norland Optical Adhesive 68, with refractive index of 1.52,and dn/dT of approximately −3.3×10⁻⁴/° C., preferably deposited usingthe standard polymer layer deposition procedure, with the preferredsolvent cyclohexanone. Alternatively the core layer may be, for example,PMMA, a thermoplastic or a crosslinked material with large negativedn/dT, and alternatively deposited by another one of the many methodsknown in the art. The top surface of the core layer is substantiallyplanar, owing to the small depth of the ridge relative to the core layerthickness. For a given ridge depth, that thickness is chosen accordingto known art to provide both lateral confinement of the optical mode anda single propagating waveguide mode. For example, if the difference ofeffective refractive index between the ridge region and the regionimmediately laterally outside the ridge, herein referred to as δn, isapproximately 0.003 or less and the width of the ridge is approximately6.0 μm or less, the waveguide will only support a single transversemode, also termed the lowest-order mode, over a 100 nm range ofwavelengths. For larger values of δn and/or wider ridges, the number ofsupported optical modes increases.

[0055] In the fifth step 550, an upper cladding layer 320, preferably, apolyacrylate copolymer (such as Gelest UMS-182), with refractive indexof 1.42 and a dn/dT of approximately −4.0×10⁻⁴/° C., is deposited overthe core layer preferably to a thickness of 1.4 μm, preferably using thestandard polymer layer deposition procedure, with the preferred solventcyclohexanone. This thickness provides a good trade-off betweenabsorption loss of the optical mode due to the applicator metal on thetop surface and degree of actuation per unit activating power, as bothare decreasing functions of upper cladding layer thickness. Thepreferred thickness can be further minimized if δn between the uppercladding material and the core material is large.

[0056] Fabrication of an applicator is the sixth step 560 of thefabrication procedure. A thermo-optic applicator 360 shown in FIG. 3 isschematically depicted in greater detail in FIG. 6. The applicator ispreferably a thin-film heater that is a resistive element 660 activatedby an electric current supplied through electrical contact stripes 662and wires 670, attached to control unit 190 (FIG. 1). The applicatoroperates by converting electrical energy into heat, which is conductedto an adjacent region of the structure, raising the temperature, andthereby causing a change in the refractive index, by the thermo-opticeffect. To fabricate the thin-film heater 660, a layer of preferablynichrome thin-film resistor material, well known in the art, isdeposited on top of the upper cladding layer 620 by a known techniquepreferably D.C. sputtering, to a preferred thickness of 0.1 μm. Goodadhesion of the thin-film heater material to the surface of the uppercladding layer is essential, and that is ensured by employment of RFbias during the preferred deposition process. To fabricate electricalcontact stripes 662 that overlap the ends of the resistor material andprovide a preferred surface for wire bonds 670, a layer of electricalcontact metal, preferably gold, is deposited on top of the thin-filmheater layer in the same pumpdown, also to a preferred thickness of 0.1μm. Electron beam evaporation may alternatively be employed.

[0057] The shape of the heater is defined preferably by a standard wetetch technique known in the art, whereby photoresist is applied on thesurface of the sputtered metal layers, then exposed through a firstapplicator mask, and developed to leave a photoresist layer of thedesired shape covering the heater 660 and contacts 662 as shownschematically in FIG. 6, for protection during etching. The mask isaligned relative to alignment marks defined previously lithographicallyon the substrate, or alternatively, made during the inverted ridgeetching step 530, so as to place the applicator directly over a sectionof the otherwise permanent waveguide and preferably laterally centeredon the line of symmetry 390 as shown in FIG. 3. The structure withphotoresist is then etched using a standard commercial gold etch,followed by a standard commercial nichrome etch. Photoresist is thenapplied again on the surface, exposed through a second applicator mask,and developed to expose the nichrome layer 660 between the contactstripes 662. Gold is then etched off the exposed area with a standardcommercial gold etch, thereby producing a thin-film heater with goldcontact pads at each end.

[0058] An alternative method for fabricating the thin-film heater 660 isas follows. First the top surface of the upper cladding layer islaminated with a laser ablation material (LAM). The patterns desired forthe heaters 660 are then laser ablated into the LAM to expose the uppercladding material only in the regions desired for heaters 660. The thinfilm resistor metal is then deposited onto the entire surface of thedevice, including both the un-ablated and ablated regions. The LAM layeris then peeled off, together with the metal superposing it. Because themetal deposition step does not completely coat the vertical sidewallsopened by the laser ablation step, the metal patterns on the surface ofthe upper cladding material will remain when the LAM layer is peeledoff. The electrical contact stripes 662 are then added in the mannerpreviously described. This alternative method avoids photolithographysteps and allows patterning to be performed without using solvents orchemicals. The technique can be used to apply many different kinds ofpatterned layers to the top surface of almost any device on which alaser ablation material can be formed.

[0059] The preferred length of the nichrome thin-film heater, that isthe applicator, is in the range of 100 μm to 500 μm between the contactstripes, in the longitudinal direction of the unswitched waveguide, andits preferred width in the lateral direction is substantially the sameas the width of the ridge 310. The applicator may alternatively belonger or shorter. However, beyond some particular length, furtherincrease of heater length does not improve switch efficiency. Dependingon the angle of deflection when actuated, no light remains in the ridgeregion beyond some distance, and a portion of an applicator extendingbeyond that does not contribute to further deflection. Thus anapplicator longer than approximately 500 μm will take up too much areaand reduce the switch density in integrated applications, and mayrequire excessive electrical power for operation. An applicator that istoo short, on the other hand, will require more activation power perunit area to produce a given deflection of light and may burn orotherwise damage the polymer materials.

[0060] Wire bonds 670 may then be made by standard techniques accordingto known art in the field of micro-electronics. Gold is the preferredmaterial for the electrical contact stripes, but other electricallyconductive materials may alternatively be used such as aluminum,titanium, chromium, conductive paint, epoxy, semiconductor, other suchconductive materials, or optically transparent materials such as oxidesof indium and tin, and liquid conductors such as salt solutions.Alternate methods of making contact to the heater may be employed, suchas by probes directly on the contact pads, or wire connections tobonding pads at the outer edges of a sample by way of a lithographicallypatterned conductor network, or connections through via holes made inthe core and cladding layers to conductors deposited and patterned onthe substrate.

[0061] The applicator may additionally have other, special-purposelayers of material, not shown, between the thin-film heater and thesurface of the upper cladding layer, that are deposited and definedaccording to known art as described with reference to the heater. Thesespecial-purpose layers may serve functions such as increasing theadhesion of the thin-film heater material to the surface of the uppercladding layer; reducing the optical loss which occurs when a portion ofthe guided wave mode extends to a metallic region, such as an electricalcontact stripe or thin-film heater; and reducing the tendency toelectrical breakdown.

[0062] Chemical compatibility is a general requirement in the choice ofmaterials used in fabrication of the anti-waveguide routing structure,in order to ensure stability of the structure over time.

[0063] The thermo-optically actuated planar anti-waveguide routingstructure of FIG. 3 operates as follows. In the off-state, when noelectrical current is supplied to the applicator 360, light propagatingin the otherwise permanent waveguide passes through the section underthe applicator with minimal disturbance, and is confined laterally by aneffective refractive index difference, δn, between the ridge region andthe region immediately laterally outside the ridge. The value of δn ispreferably 0.003 or less, with the ridge or core region having a highereffective refractive index than the region immediately laterally outsidethe ridge, for a normal waveguide mode.

[0064] When the applicator is activated, that is, electrical current issupplied to the applicator from control unit 190, the nichrome thin-filmresistor heats up and raises the temperature of the solid materialsadjacent to, and physically near it. The applicator 360 is shown in FIG.3 to have limited width in the lateral direction, and it preferably hassubstantially the same width as the ridge 310. According to well-knownprinciples described for example in Carslaw and Jaeger, Conduction ofHeat in Solids, incorporated by reference herein, a temperature gradientdevelops, such that the temperature is high nearest to the applicator,decreases farther away, and eventually, far enough away from theactuator, reaches a normal value that is nearly the temperature in theoff-state. Owing to the temperature gradient, the entire core regionbeneath the electrode (as shown, the ridge region of the core) which iscloser to the applicator, heats to a higher temperature than the regionimmediately laterally outside the ridge, which is situated farther fromthe applicator.

[0065] As a consequence of the thermo-optic effect, the refractive indexof the portions of the materials that are at a higher temperatureacquire a larger index change relative to values in the off-state at anormal temperature. The amount and direction of change depends on thethermo-optic coefficients of the materials. For most polymers the valueof dn/dT is negative and between −0.8×10⁻⁴/° C. and −5.0×10⁻⁴/° C., andfor many glasses it is between 0 and positive 10⁻⁶/° C., but may also benegative. The core material has preferably a large negative value ofdn/dT. The cladding layers are chosen to have refractive indices lessthan the core layer, and in particular, the difference in refractiveindex between the top cladding and the core is preferably large(approximately 0.1) to minimize the insertion loss from the applicator.The hotter ridge region of the core thus experiences a large decrease inrefractive index and the less hot region immediately laterally outsidethe ridge, a smaller decrease, thereby creating an anti-waveguidestructure in the heated portion of the otherwise permanent waveguidestructure, with a reversed, negative value of δn. A smaller value ofdn/dT for the lower cladding layer further limits the decrease inrefractive index produced in the region immediately laterally outsidethe ridge, where the layer is thicker, and thus helps to create a givennegative value of δn with the least degree of actuation. When theapplicator is activated to a sufficient degree as described withreference to FIG. 4, light propagating in the waveguide toward theapplicator becomes redirected, as it enters in and travels in theanti-waveguide section formed under the applicator, into two paths oneither side of the ridge, while remaining substantially verticallyconfined in the plane of the core layer 340. The degree of actuationaffects the value of δn, and that determines the strength ofanti-waveguiding produced in the structure in terms of the sharpness orthe angle of light deflection. The expected deflection angle of light inthe preferred embodiment as a function of the lateral difference ofeffective refractive index, δn, is shown in FIG. 7, based on analysis ofthe waveguide structure by the beam propagation method known in the art,which method is described for example in the Marcuse referenceincorporated above, at pp 306-318. With reference to FIG. 7, if thenormal (unswitched) value of δn is 0.003, a change of −0.010 uponactuation may be required, in order to overcome normal confinement andproduce light deflection at an angle of 4 degrees, for example, in theembodiment of this invention.

[0066] Vertical confinement is obtained in the preferred planarembodiment by having a large refractive index difference between thecore and the cladding layers in the off-state, preferably 0.100 betweencore and upper cladding layer, and 0.030 between core and lower claddinglayer, such that the structure remains vertically confining, that is,core larger than cladding, also while actuated to the maximum expecteddegree.

[0067] Note that whereas anti-waveguiding can be initiated by thespecific methods described above, many variations are possible as well.All that is required is that in one state, a portion of the confinementregion have a higher index of refraction than an outside region adjacentto the confinement region, by a difference that is sufficient to confinethe optical mode in the direction of the outside region; and that in thesecond state a portion of the outside region adjacent to the confinementregion now have an index of refraction that is higher than that of aportion of the confinement region adjacent to the outside region. In oneembodiment, the switching from the first state to the second state canbe accomplished by depressing the index of the confinement regionwithout changing the index of the outside region, or by depressing theindex of the confinement region while increasing the index of theoutside region, or by depressing the index of both regions but doing soto a greater extent in the confinement region than in the outsideregion. The system can change the index in the confinement region to adifferent extent than in the outside region either through the use oftwo different materials in the two regions (e.g. a core material in theconfinement region and a different cladding material in the outsideregion), or by using a single material in both regions (or differentmaterials with similar coefficients of index change per unit of controlsignal applied) but applying the control signal preferentially in oneregion or the other. For example, a thermo-electrode applicator (heatingelement) can be formed to superpose the confinement region and not theoutside region. As an alternative example, where the material of atleast the outside region has a positive electro-optic coefficient,applicators (electrodes) opposing a common ground plane electrode can beformed superposing part of the outside region adjacent to theconfinement region but not superposing the confinement region.

[0068] An asymmetrical planar embodiment of the invention has anasymmetrically positioned applicator 860, as shown in plan view in FIG.8, but it is otherwise substantially similar to the planar embodimentdescribed with reference to FIGS. 1 and 2. The materials are chosen suchthat the values of the thermo-optic, or alternatively electro-optic,coefficients of the core and side cladding layers are reasonablymatched. The applicator 860 is on the top surface of an upper claddinglayer generally above a core 810, which is shown in dashed lines. Theapplicator is shown to be wider than the core and aligned on one sidewith the core but overlapping some distance on the other side, over aside cladding region. The figure also shows a cross-hatched extensionregion 866, but this region is not necessary in this embodiment.Activation of the asymmetrically positioned applicator, by means of thethermo-optic or alternatively by other effects, creates ananti-waveguiding region primarily to one side of an otherwise permanentsymmetrical waveguide structure, while the structure remains confiningto light on the opposite side and also vertically. This may beunderstood with reference to the description given above of theoperation the symmetrical planar embodiment of the invention. Uponactuation, owing to an asymmetrically positioned and wider applicator,the part of the right side cladding adjacent to the core, that isdirectly under a part of the applicator, is situated substantially thesame distance from the applicator as the portion of the core 810 underthe applicator, and therefore heats up substantially to the same degree.As the thermo-optic coefficient values of the core and side claddinglayers are reasonably matched, the refractive index decrease is thussubstantially the same in the regions, and that substantially preservesthe prior value of the refractive index difference between core andright side cladding adjacent to the core, which also preserves thenormal lateral confinement on the right side, in the evanescent tailregion of the prior optical mode of the otherwise permanent ridgewaveguide. On the left side, however, the refractive index differencebetween core and cladding, and the consequent strength ofanti-waveguiding created, is substantially similar to the symmetricalcase, described with reference to FIGS. 3, 4, 5, and 6. An asymmetricalstrength of lateral confinement of light is thus produced, and lightfrom the unswitched waveguided path indicated by an arrow 881 isaccordingly forced preferentially to one side, into a second path in thehorizontal plane indicated by an arrow 882, away from the applicator.

[0069] In order to simplify the description of the invention, as usedherein, “directions of confinement” are limited to directions orthogonalto the normal (unswitched) propagation direction of an optical mode.Thus a planar optical waveguide confines an optical mode in both theupward and downward directions but not in either of the two oppositelateral directions. A waveguide formed with top, bottom, left and rightside cladding materials, confines an optical mode in all directionsorthogonal to the direction of mode propagation. Similarly, ifconfinement in a given direction is suppressed, such as by turning onthe optical switch, then the direction to which optical energy escapesthe confinement region is referred to herein as the “given direction,”even though the centroid of the escaping optical energy follows a paththat may diverge only slightly from the unswitched propagation directionof the mode. Referring to FIG. 8, therefore, confinement is maintainedin the switched state in the rightward direction 890, but suppressed(actually more than suppressed) in the leftward direction 892.

[0070] An asymmetrically positioned applicator alternatively can havenon-uniform width, such as a curved left side, as indicated by thecross-hatched extension 866 of the applicator 860 shown in FIG. 8. Theeffect of such a curved side as shown will be to cause a greaterdeflection of light to the left, upon actuation, as the deflected beam882 continues to encounter the anti-waveguiding refractive indexboundary between the core and cladding on the left side, for a greaterdistance than without the extension. The non-uniform width is shown as acurved shape but linear tapers and other non-uniform shapes mayalternatively be employed. Note also that the input and output edges ofthe thermo-optic applicator 860 remains perpendicular to the direction881 of light propagation along the unswitched path, to ensure minimalinsertion loss.

[0071] Another alternate asymmetrical planar embodiment has asymmetrical applicator but asymmetrical confinement of light in theotherwise permanent waveguide structure, such that there is weakconfinement laterally only on one side of the core and strongerconfinement on the other side. Such asymmetrical confinement may beobtained by appropriate choice of the refractive indices of the sidecladding layers 140 and 150, and the core 110, as shown and describedwith reference to FIGS. 1 and 2. For example, weaker confinement isobtained on the left side if the refractive index difference between 110and 140 is 0.003 or less, and stronger confinement is obtained on theright side if the refractive index difference between 110 and 150 is0.030, in the otherwise permanent waveguide structure. Alternatively theasymmetrical confinement may be realized in a ridge waveguide structureby asymmetrical thickness of a slab core layer laterally outside theridge. Upon actuation, heating is produced symmetrically in thestructure as described for the symmetrical embodiment, but the value ofthe consequent lateral negative difference of refractive index betweenthe core and side cladding near the core is sufficient to createanti-waveguiding only in the horizontal plane on the weakly confinedside, and not the opposite side, thus deflecting light from the priorwaveguide mode, or first path, into a second path, toward the weaklyconfined side.

[0072] The asymmetrical planar structures can produce a single-lobedlight deflection pattern, with an intensity profile substantiallysimilar to one-half of the double-lobed profile shown for thesymmetrical case in FIG. 4.

[0073] The terms “right” and “left” as used herein refer to oppositedirections in a relative sense and may be interchanged, or rotated withrespect to a fixed frame of reference, within the teaching of thisinvention.

[0074] The preferred asymmetrical planar embodiment is a ridge waveguidestructure with asymmetrically positioned thermo-optic applicator. Thepreferred embodiment is described with reference to FIG. 8, with orwithout the extension portion 866, where the dashed lines 811 and 812are understood in the present context to represent the edges of theridge of an otherwise permanent ridge waveguide structure. The structureis shown more clearly in a cross-sectional view, FIG. 9, taken along aline C-C identified in FIG. 8. Referring to the figures, there is showna thermo-optic applicator 960 positioned predominantly to one side ofthe ridge 910, preferably so that its left edge 961 is verticallyaligned with the left edge of the ridge, denoted by a vertical line 911that intersects 811. The right edge of the ridge is denoted by avertical line 912 that intersects 812. The applicator is wider than theridge, as shown, with its right edge 962 extending beyond 912 preferablyto a distance substantially equal to the width of the ridge.

[0075] In respects other than the positioning and width of theapplicator, the preferred asymmetrical embodiment is substantiallysimilar in construction to the thermo-optically actuated (symmetrical)planar anti-waveguide routing structure described with reference to FIG.3. The operation of the preferred embodiment is substantially asdescribed above for an asymmetrical planar embodiment, after firstreference to FIG. 8, with the exception that references to “core” shouldbe substituted by “ridge region”; “side cladding adjacent to the core”,by “region immediately laterally outside the ridge”; and “refractiveindex difference between core and side cladding adjacent to the core”,by “δn”.

[0076] Alternatively in the thermo-optically actuated embodiments ofthis invention, including asymmetrical embodiments, an applicator may besituated under a core and separated from it by a lower cladding layer.For example with reference to FIG. 3 an applicator 360 may be situatednot on top of the upper cladding layer 320 but, instead, under a ridge310 and a lower cladding layer 330, and on top of a substrate, notshown. Suitable modification of the lower cladding layer thickness, andof the fabrication procedure as described herein with reference to FIG.5, will in that case be required as apparent to those skilled in theart.

[0077] Actuation by means of the electro-optic effect may alternativelybe used in the invention. Electro-optic actuation may be beneficial forapplications that require faster switching or light redirection thanobtainable with thermally activated switches. This will require suitablemodification of the applicator to apply an electric field to thematerials of the anti-waveguide routing structure, upon activation by anapplied voltage, and, further, modification of the choice of materialsto provide appropriate electro-optic coefficients such that, uponactivation of the applicator, the refractive index difference between atleast a portion of the core and the adjacent surrounding cladding willbecome negative.

[0078] Referring to FIG. 10 there is shown in cross-sectional view anelectro-optically actuated planar embodiment of the invention,comprising a ridge waveguide structure with upper cladding layer 1020,lower cladding layer 1030, an slab core layer 1040, a ridge 1010 that isa thicker section of the core layer that serves to define theconfinement region and provide lateral confinement of light according toknown art, as described with reference to FIG. 3, an upper electrode1061, and a ground plane electrode 1062, fabricated on a substrate 1050.

[0079] The structure may be fabricated by depositing an electricallyconductive metallic layer that forms the ground plane electrode 1062, ona substrate 1050, by a known process such as sputtering, oralternatively, by evaporation. A lower cladding 1030 with refractiveindex less than the core layer 1040 is deposited on the metallizedsubstrate by a known process, such as spinning. An electro-opticmaterial that forms the core layer 1040 is then deposited to a thicknessthat allows at least single mode propagation to occur. A layer ofphotoresist is deposited on the electro-optic core layer 1040, exposedthrough a mask, and developed to leave a protective stripe pattern withthe desired width of the waveguide ridge 1010. The width is selectedaccording to known art to achieve single mode operation upon completionof the structure, as described with reference to FIG. 3. A thickness ofthe core layer 1040 is removed in the unprotected area, by a knownprocess such as laser ablation, or alternatively by RIE or another knownetching process, to fabricate a ridge 1010. The photoresist stripepattern is removed and a low-refractive index upper cladding layer 1020deposited. The upper and lower cladding layers 1020 and 1030 are madesufficiently thick to prevent evanescent optical mode tails fromextending significantly into the electrodes 1061 and 1062. The structureis then electric field poled using a corona poling process, oralternatively another electric field alignment process such as contactpoling, as known in the art and described for example in Burland, D. M.Miller, R. D., and Walsh, C. A., Chemical Reviews 94, 31-75 (1994),incorporated herein by reference, to induce second-order nonlinearoptical properties and consequent electro-optic response when subjectedto an electric field. Some nonlinear optical materials that canalternatively be suitable for the core layer 1040, such as thoseintrinsically lacking inversion symmetry, such as Langmuir-Blodgett orionic self assembled films, may not require electric field poling inorder to have electro-optic response when subjected to an electricfield. A metal film is then deposited and patterned to form an upperelectrode 1061, in substantially the same manner as described to formthe ridge 1010, excepting that a metal etch is employed to remove all ofthe metal layer in the unprotected area. The upper electrode preferablyhas substantially the same width as the ridge and is preferably alignedto be directly above it, as indicated by the vertical dashed lines 1091and 1092, and its length in the otherwise permanent waveguide directionis limited. Electrical contacts such as wire bonds 1070 are made to theelectrodes 1061 and 1062. Access to the ground plane electrode 1062 maybe provided by known means used in semiconductor and microelectronicspackaging, such as a via opening formed through the waveguide layers,that may be selectively metal coated and/or electroplated as indicatedschematically by 1075.

[0080] In operation, the electro-optic applicator is activated by acontrol voltage from control unit 190 (FIG. 1), applied between theelectrodes through the contact wires 1070, thereby modifying therefractive index of a portion of the core material in proximity to theupper electrode 1061. Upon application of a voltage with a bias reverseto that of the original poling field, the refractive index will decreaseproportionally to the electro-optic coefficient and the applied voltage,which determines the degree of actuation. At a sufficient voltage theeffective refractive index of the portion of the ridge region 1010 thatis under 1061 will decrease below that of the region immediatelylaterally outside the ridge, and a segment of anti-waveguide will beformed. Light propagating in the waveguide, toward the applicator,becomes redirected while traveling in the anti-waveguide section underthe applicator into two paths on either side of the ridge, whileremaining substantially vertically confined in the plane of the corelayer 1040. Other than the applicator and the mechanism for creating arefractive index change, the operation of the electro-optically actuatedridge waveguide embodiment is substantially similar to that of thethermo-optically actuated embodiment, described above with reference toFIGS. 3, 4, and 7. The length of the upper electrode 1061 is chosen tobe sufficient to provide deflection of light without coupling back intothe waveguide section beyond the upper electrode, for a given degree ofactuation.

[0081]FIG. 11 is a cross-sectional view illustrating an alternativeelectro-optically actuated planar embodiment. The structure is the sameas that of FIG. 11, except that the core layer 1040 has been replaced bya different material 1140 which has a positive electro-opticcoefficient, and the upper electrode 1061 over the ridge 1010 has beenreplaced by two upper electrodes 1161 and 1162 laterally outside theridge 1010, adjacent to the ridge on either side thereof.

[0082] In operation, the electro-optic applicators are activated by acontrol voltage from control unit 190 (FIG. 1), applied between theelectrodes 1161 and 1162, on the one hand, and ground plane 1062 on theother hand, through the contact wires 1070, thereby modifying therefractive index of the portions of the core material under each of theelectrodes 1161 and 1162. Upon application of a voltage with a biassimilar to that of the original poling field, the refractive index inthese regions laterally outside the ridge region 1010 will increaseproportionally to the electro-optic coefficient and the applied voltage.At a sufficient voltage the effective refractive index of the portionsof the core layer that are under the electrodes 1161 and 1162 willincrease above that of the ridge region 1010, and a segment ofanti-waveguide will be formed. As in the embodiment of FIG. 10, lightpropagating in the waveguide toward the applicator, will becomeredirected while traveling in the anti-waveguide section between theapplicators 1161 and 1162 into two paths on either side of the ridge,while remaining substantially vertically confined in the plane of thecore layer 1040.

[0083] In applications where lower optical loss is desirable, astrip-loaded waveguide structure may be employed in place of the ridgewaveguide structure shown in FIG. 10. The strip-loaded structure is aknown type of ridge waveguide wherein the ridge is made of a differentmaterial than the extensive slab core layer. In a ridge waveguide andalso in the strip-loaded structure, most of the light in a normaloptical mode, as denoted by 380 in FIG. 3, travels in the extensive slabpart of the core layer as indicated.

[0084] Accordingly, an alternate electro-optically actuated embodimentof the invention comprises a strip-loaded waveguide structuresubstantially similar to the electro-optically actuated embodimentdescribed with reference to FIG. 10, with the exception that the corelayer is an extensive slab waveguide made of an optical material thathas low loss but no significant electro-optic properties, upon which isdeposited a strip material that has significantly large electro-opticproperties but can have higher optical loss. The strip material ispatterned and formed into a shape similar to a ridge, thereby providingwaveguide confinement laterally according to known art, as describedwith reference to the ridge waveguide and FIG. 3. The strip material ispreferably a layer of electro-optic polymer with thickness in the rangeof 0.1 to 0.5 μm and a refractive index slightly greater than that ofthe upper cladding. In the resulting strip-loaded waveguide structure,most of the light in a normal optical mode travels in the low-lossoptical material, and consequently the strip-loaded waveguide willexperience less optical loss than a comparable ridge waveguide with anelectro-optic core. A further desirable feature of the structure is thatit is less sensitive to excessive width and spatial misalignment of theupper electrode to the strip ridge, as the electro-optic coefficient islarge only in the strip ridge and not in the other parts of thestructure. Consequently the refractive index of a slab core regionimmediately laterally outside the ridge, that may be directly under apart of the upper electrode, will not be changed upon actuation, and thestructure will respond substantially equivalently to a ridge waveguidestructure with a more accurately fabricated upper electrode.

[0085] Yet another alternate electro-optically actuated embodiment hasan asymmetrically positioned upper electrode but is otherwisesubstantially similar to the embodiment described with reference to FIG.10, which employs a ridge waveguide structure. In the asymmetricalelectro-optic embodiment the upper electrode is positioned to one sideof the center line of an otherwise permanent ridge waveguide whileremaining parallel to and partially overlapping the ridge region as wellas an adjacent region immediately laterally outside the ridge, whichboth have substantially the same electro-optic coefficient. Thus uponactuation to a sufficient degree, a weakly confining δn is preserved onthe overlapping side but an anti-waveguiding region is created on theopposite side of the waveguide, away from the upper electrode. Otherthan the applicator and the form of activation (how a refractive indexchange is created), the operation of the electro-optically actuatedasymmetrical ridge waveguide embodiment of the invention issubstantially similar to that of the thermo-optically actuatedasymmetrical ridge waveguide embodiment, described above with referenceto FIGS. 8 and 9.

[0086] An electro-optic applicator in the form of a ground planeelectrode and a narrow upper electrode of limited length has beendescribed, but other physical arrangements of the applicator, shape ofthe cladding surfaces upon which it rests, and connecting means mayalternatively be used.

[0087] Another preferred embodiment is a vertical version of theinvention employing a thermo-optically actuated structure, that may bedescribed with reference to FIG. 2. The operation of this firstpreferred vertical embodiment is substantially similar to that of theprevious embodiments. An activation current is applied to the applicatorsituated substantially over the waveguide. The current flow in theresistive applicator generates heat that flows through the opticalwaveguide structure toward cooler regions farther from the applicator.The optical waveguide structure is fabricated preferably from polymermaterials wherein an increase in temperature results in a decrease inrefractive index, by the thermo-optic effect.

[0088] Heat flows from the applicator towards the substrate whichpreferably acts as a heat sink. The heat flow through the structure isassociated with a temperature gradient through the material. It will beappreciated that an alternative or additional heat sink may be employedto enhance the thermal gradient, a heat sink fabricated from a glass orother material having a relatively high thermal conductivity so as toefficiently remove heat. The temperature gradient produces, through thethermo-optic effect, a refractive index varying with depth beneath theapplicator. This is shown schematically in FIG. 15 for an opticalwaveguide structure composed of materials with matched, or substantiallyequal, dn/dT values. In this embodiment the core layer of the waveguidepreferably has a negative dn/dT that is substantially similar to that ofthe cladding layers such that the same temperature rise results in asubstantially similar decrease in refractive index in each layer.Temperature at different distances from the applicator along a verticalline such as 290 shown in FIG. 2, is indicated by curve 1510 in FIG. 15,and the upper and lower interfaces of the core with the respectivecladding layers are denoted by dashed lines 1520 and 1530, respectively.

[0089] The overall refractive index profile of the waveguide structurewith and without activation is indicated by the curves 1550 and 1540,respectively, in FIG. 15. Without activation it is clear that thestructure is capable of acting as an optical waveguide with verticalconfinement of light. However, with the degree of activation shown inthe figure, the optical waveguide is effectively erased and therefractive index of a portion of the core layer (in particular theshallowest portion of the core layer) is reduced below that of theadjacent lower cladding layer. The temperature gradient within theoptical waveguide structure must be steep enough to provide asignificant temperature difference between the upper and lowerinterfaces of the core layer, such that the refractive index of theupper interface of the core is decreased substantially below that of thelower interface of the core. Although there is still a refractive indexstep at the boundary between the core and cladding layers, therefractive index of at least a part of the core is reduced below that ofthe lower cladding layer, enabling light to tunnel through the indexbarrier into the higher index of the lower cladding layer, thus creatingan anti-waveguide region beneath the applicator.

[0090] At the degree of activation resulting in a refractive indexprofile as shown in FIG. 15, the light propagating in the otherwisepermanent waveguide before the anti-waveguide section is preferentiallydeflected towards and into the lower cladding layer, in theanti-waveguide section, by a combination of diffraction and refraction,as a result of both removal of waveguide confinement in a directionvertically away from the applicator and a relatively higher refractiveindex in the lower cladding layer compared to the core layer.

[0091] In the first preferred vertical embodiment the light is notdeflected in the lateral or horizontal direction because actuationpreferably does not substantially affect waveguide confinement in thelateral direction. The thermal or temperature gradient in the lateral,or horizontal, direction is arranged to be significantly smaller than inthe vertical direction, as controlled by the width of the applicator. Awide applicator, that is, for example, one with a width wider than thewaveguide core, can provide a substantially uniform temperature profileacross the core and laterally adjacent cladding regions, resulting insubstantially similar refractive index decreases within the laterallyadjacent regions, thus preserving lateral waveguide confinement.

[0092] The first preferred vertical embodiment provides for efficientdeflection of light from an otherwise permanent waveguide into apreferential direction away from the applicator, in a structure thatoperates in a manner substantially independent of ambient temperaturevariations. Because of substantially similar values of dn/dT in thepolymer layers comprising the structure, a change of ambient temperatureproduces only a uniform temperature change throughout the structure andthus does not change the shape of the refractive index profile,resulting in a refractive index structure and waveguide confinement thatare substantially independent of the ambient temperature of the device.The operation of the device, on the other hand, relies on creation of avertical temperature gradient through the waveguide core.

[0093] In a second preferred vertical embodiment, vertical deflection oflight from the otherwise permanent waveguide is achieved by constructingan optical waveguide structure that has stronger confinement in thelateral (horizontal) direction than in the vertical direction away fromthe applicator. This may be provided by depositing side cladding regionswith a lower effective refractive index than the core region in astructure as shown in FIG. 2. It is preferably provided by a ridgewaveguide structure as shown in FIG. 3 wherein the ridge depth is chosensuch that the structure remains laterally confining, that is, ridgeregion 310 of the core layer 340 having a larger effective refractiveindex than the region immediately adjacent to and outside the ridge,also while actuated to the maximum expected degree. Such ridge depth maybe significantly larger than the ridge depth for the planar embodimentsdescribed hereinabove with reference to step 530 of FIG. 5. In the limitof a ridge etched all the way through the core layer, the structurebecomes a rectangular, or square-profile, channel waveguide coresurrounded above and laterally by an upper cladding layer 320. The uppercladding layer 320 is chosen to have a refractive index value smallerthan that of the core layer, to provide strong confinement of lightvertically in the direction towards the applicator.

[0094] The lower cladding layer 330 is chosen to have a refractive indexvalue only slightly less than that of the core layer, thereby providingweak confinement vertically away from the applicator.

[0095] The values of dn/dT in this embodiment may be different in thedifferent layers, provided that the light confinement remains asdesired, throughout the specified ambient temperature range of thedevice for a given application.

[0096] The operation of the second preferred vertical embodiment issimilar to that of the said first, and may be explained also withreference to FIG. 15, with references to the core being here understoodto mean the ridge region, and the distance from the applicator beingmeasured along a line 390 as shown in FIG. 3. In operation when theheater is actuated, a temperature gradient is created whereby a portionof the ridge region under the applicator, being closer to theapplicator, is hotter than the portion of the lower cladding layersituated adjacent to and immediately vertically below the ridge.Consequently, owing to a large negative thermo-optic coefficient of thecore material, an anti-waveguide structure is created in the otherwisepermanent optical waveguide section under the applicator, in thevertical direction away from the applicator, when actuated to asufficient degree. Light propagating in the normal waveguide sectionbefore the applicator is consequently forced to deflect out of the ridgeregion in a vertical direction away from the applicator.

[0097] Self-aligned fabrication may be employed to construct ananti-waveguide routing structure of this invention, whereby the samepattern of material is used to form both an optical waveguide and anapplicator. This provides more accurate positioning of the applicatordirectly above a section of the waveguide, than a construction methodwherein the parts are formed separately by two successive patterningsteps, with alignment in between.

[0098] An embodiment of the invention employing an optical channelwaveguide, as depicted in FIGS. 1 and 2, may be constructed by aself-aligned fabrication method in which the waveguide is photodefined.The self-aligned photodefined method is set forth in the flow chart ofFIG. 12, and may be most easily understood as a modification of thepreferred procedure described with reference to FIG. 5. Referring toFIG. 12, the self-aligned photodefined fabrication procedure begins instep 1210 with adhesion enhancement of the substrate surface. In step1220, the lower cladding layer is deposited on the substrate, and instep 1240, a core layer is deposited. In step 1250, the upper claddinglayer is deposited above the core layer. Steps 1210, 1220, 1240 and 1250are the same as described above with respect to steps 510, 520, 540 and550 in FIG. 5, but note that the step 530 of defining and etchinginverted ridges on the lower cladding is omitted. Thus after step 1250the procedure has created a preliminary unpatterned article.

[0099] In step 1260, applicator material, patterned initially to matchthe desired pattern of waveguides in the body of the article, is formedon the top cladding layer. The applicator material can be either aheater material (for thermo-optic embodiments) or electrode material(for electro-optic embodiments), for example. It can be formed in anynumber of conventional ways, such as by coating the top surface of thetop cladding with the material and patterning it lithographically. Thepattern of the applicator material formed in step 1260 typicallyincludes stripes of the width desired for the ultimate waveguides in thebody of the article, and extending longitudinally along the desiredwaveguide direction.

[0100] In step 1262, the waveguide pattern is defined into the body ofthe article using the patterned applicator layer as a mask in a mannerhereinafter described. Subsequently, in step 1270, the applicator layeris further patterned to delimit the length of applicator segments in thewaveguide direction to the lengths desired for the final applicatorpattern, and to make electrical contacts.

[0101] The step 1262 of defining the waveguide pattern into the body ofthe article can be accomplished in several different ways. In a firstembodiment, the core material is a photo-bleachable polymer known in theart, such that its refractive index becomes permanently decreased as aresult of irradiation with light of suitable wavelength, for suitabletime duration, in the area unprotected by the opaque applicator stripe.The refractive index of the core layer under the stripe remainsunaffected and thereby forms the higher-refractive index core of arectangular channel waveguide that has a width substantially equal toand well aligned with the applicator stripe.

[0102] In an alternate, second embodiment, which may be referred to asan etch-and-refill waveguide process, instead of illuminating thepatterned area, an RIE process such as that described above with respectto step 530 (FIG. 5) can be employed to etch through the upper claddinglayer and also a predetermined distance into the core layer, to form aridge structure. If a photoresist was used to pattern the applicatorstripes, the same photoresist can be retained on the applicator stripesto protect them against the RIE process. After etching, top claddingmaterial is re-applied to the structure. Preferably, the top claddingmaterial is re-applied first to a greater thickness to provide asubstantially planarized surface, and then thinned by RIE to theoriginal application thickness of step 1250, thereby also exposing theapplicator stripe. In some embodiments it will be acceptable to leave apolymer on top of the applicator metal.

[0103]FIG. 13 illustrates yet another variation on the self-alignedfabrication method, using a negative photoresist and known lift-offprocess. In this embodiment, steps 1210, 1220, 1240 and 1250 are thesame as in FIG. 12. Instead of forming applicator material as in step1260, however, a light-blocking lift-off material is formed andpatterned to inversely match the waveguide pattern desired for the bodyof the article (step 1360). The lift-off layer can be formed by firstapplying a lift-off polymer, then a light blocking material layer, andfinally a negative photoresist material. The stripe pattern is thenexposed through a mask and developed, leaving the stripe area open andthe rest of the area protected by hardened photoresist. The lightblocking material layer is etched out in the stripe area, and thelifting layer is also removed by a suitable etching means such as RIE.Alternatively a positive photoresist and an inverse stripe mask may beemployed. As yet another alternative, a suitable material may serve as acombined negative photoresist, light blocking layer, and liftingmaterial, simplifying the process as will be apparent to those skilledin the art. With the lift-off process, the applicator stripe area isopen, while the rest of the surface is protected by the light blockingmaterial on top of the lifting layer, or alternatively by the liftinglayer that is also a light blocking layer.

[0104] In step 1362, the waveguide pattern is defined into the articleby irradiating the article through the upper cladding layer withultraviolet light, or light of another suitable wavelength range. Thecore material is a known glass, or alternatively a polymer material,whose refractive index becomes permanently increased as a result ofirradiation with ultraviolet light, or alternatively, with light ofanother suitable wavelength range. Such glass material and irradiationprocess are known and used to make fiber Bragg gratings in optical fiberfor application in fiber optic communication networks. See alsoChandross U.S. Pat. Nos. 3,809,732, 3,809,696, 3,993,485, and 3,953,620,all incorporated herein by reference. The upper cladding layer istransparent to, and unaffected by the light. The refractive index of thecore layer in the rest of the structure thus retains its original value,while the stripe area becomes a higher-refractive index confinementregion of a rectangular channel waveguide.

[0105] In step 1364, after irradiation, applicator material is depositedon the structure, both on the upper cladding layer, within the openstripe, and on the metal and lifting layer, elsewhere. In step 1366 thelifting layer is then dissolved with suitable agitation, by a knownprocess, thereby lifting off the applicator material in the area outsidethe stripe and forming the applicator stripe that has a widthsubstantially equal to and well aligned with the waveguide confinementregion. Further patterning of the applicator proceeds as described forthe other self-aligned processes (step 1370).

[0106]FIG. 14 is a plan view of an asymmetric planar optical switchaccording to the invention, in which optical energy that escapes theconfinement region 1410 when the device is in its “on” state follows apath 1412 through the planar waveguide toward a target structure 1414.The particular embodiment of FIG. 14 uses a thermo-optic ridge waveguide1416 below the top cladding layer 1418, and an asymmetrically positionedheating element 1420 superposing the full width of a particular lengthof the ridge, and extending by an equal width laterally beyond theright-hand edge of the ridge. However, any switching structure describedherein can be used. The target structure 1414 can be a detector, forexample, or an out-of-plane deflector. Other examples include displaypixel structures, and the input end of a secondary output waveguide. Thetarget 1414 is preferably spaced from the unswitched waveguide structure1416 by some distance to ensure that it does not significantly interferewith the evanescent tail of the optical energy propagating therein whenthe switch is off. In addition, the target 1414 is also disposed at theproper lateral distance from waveguide 1416 to capture the largestpossible fraction of the optical energy escaping along path 1412 fromthe confinement region 1410. That position is laterally farther awayfrom the unswitched waveguide 1416 (for a given longitudinal positionalong waveguide 1416) where the switch is operated in an anti-waveguideon-state, than it would be if the switch were operated merely tosuppress waveguiding, because in the former case escaping light divergesfrom the unswitched waveguide 1416 due to refractive as well asdiffractive effects, whereas in the latter case it diverges due only todiffractive effects. Light in the former case therefore diverges fromthe unswitched waveguide 1416 at a greater angle.

[0107] As used herein, the terms “above” and “below” are intended to beinterpreted transitively. That is, for example, if layer A is abovelayer B which is above layer C, then layer A is also above layer C.

[0108] Also as used herein, a given event is “responsive” to apredecessor event if the predecessor event influenced the given event.If there is an intervening processing step or time period, the givenevent can still be “responsive” to the predecessor event. If theintervening processing step combines more than one event, the output ofthe step is considered “responsive” to each of the event inputs. If thegiven event is the same as the predecessor event, this is merely adegenerate case in which the given event is still considered to be“responsive” to the predecessor event. “Dependency” of a given eventupon another event is defined similarly.

[0109] The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art.For example, all embodiments shown or described herein using one form ofactivation (thermo-optic, electro-optic, acousto-optic ormagneto-optic), can instead be constructed using any of the other formsof activation, provided that materials are available with the desiredpositive or negative activation coefficients and other desired optical,mechanical and chemical properties. Also note that anti-waveguiding isnot necessarily required in asymmetric planar case. Also note that inthermo-optic embodiments, the core material may be any material having anon-zero thermo-optic coefficient. Alternative materials to thosespecified herein include semiconductors such as Si, or GaAs, glass, andother amorphous materials, and crystals such as LN. Furthermore, andwithout limitation, any and all variations described, suggested orincorporated by reference in the Background section of this patentapplication are specifically incorporated by reference into thedescription herein of embodiments of the invention. The embodimentsdescribed herein were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A method for switching an optical switch from a first state to asecond state, said optical switch including a structure which in saidfirst state provides confinement against propagation of an optical modein at least a first direction out of a confinement region, saidconfinement region having a negative thermo-optic coefficient,comprising the step of heating at least part of said confinement regionuntil at least a first part of said confinement region has a refractiveindex which is below the index of refraction of a first region of saidswitch outside of and adjacent to said confinement region in said firstdirection.